CELL DIVISION

Stages of mitosis (animal cell) prophase: - chromosomes condense (replicated in S phase) - centrosomes separate (duplicated in S phase) prometaphase: - nuclear envelope breaks down - MTs contact chromosomes, spindle forms

metaphase: - chromosomes align at spindle equator (metaphase plate) anaphase: - sister chromatids separate - chromosomes move to poles - poles move apart telophase: - nuclear envelope reforms - chromosomes decondense - interphase array of MTs reforms cytokinesis: - contractile ring pinches cell in two

Centrosome cycle (animal cell): - centrosome (centriole) duplication begins at the start of S phase - remains as one complex until M phase In early embryonic cells, the centrosome cycle can operate without a nucleus - egg cell extracts

Mediators of mitotic chromosome structure: - cohesins: deposited along the length of sister chromatids as the DNA is replicated - hold sisters together - condensins: coil DNA - mediate chromosome condensation

Structure of a spindle: 3 classes of MTs (polar MTs) How does a spindle form, and how does it work to separate chromosomes?

Prophase: changes in MT dynamics - more MTs nucleated from centrosome - shorter, more dynamic MTs Quantifying MT dynamics: - inject fluorescent tubulin - bleach with laser - measure recovery (newly formed MTs) - t1/2 = time to 50% recovery

MT dynamics: regulated by MAPs vs. catastrophins higher catastrophe rates = shorter MTs centrosomes incubated in Xenopus egg extracts

Spindle formation in vitro: mitotic extracts + DNA + centrosomes - abnormal spindles form when ratio of MAPs:catastrophins is perturbed no MAP (MTs are too short)

Centrosome separation in prophase is driven by plus-end motors (KLPs) - balanced by minus-end motors

Yeast mutants: identification and characterization of spindle motors (-) (+)

Prometaphase: kinetochores capture MTs (mechanism of attachment??)

Forces that drive chromosomes to metaphase plate: - kinetochores pull chromosomes to poles: (-)end directed motors? - astral ejection force: (+)end directed motors on chromosome arms

Metaphase: - chromosomes continue to oscillate at metaphase plate (vertebrate cells) - MTs undergo poleward flux (function?)

- poleward flux of metaphase MTs can be measured with caged fluorescein

Dynamics of individual MTs can be measured with fluorescence speckle microscopy: - poleward flux of metaphase MTs occurs in kinetochore and overlap MTs but not in astral MTs

Anaphase A: - kinetochore MTs shorten - chromosomes move to poles Fluorescent tubulin injections show locations of MT growth, depolymerization

Two models for kinetochore movement along MTs:

Anaphase B: - poles separate - overlap MTs lengthen

Model for motor protein activity in anaphase B

Bipolar spindles can assemble without centrosomes or chromosomes

Cytokinesis (animal cell)

What determines the position of the cleavage furrow? - signal from asters to cortex - signal from central spindle - chosen before mitosis (position of spindle from previous mitosis)

Contractile ring of actin and myosin: red = actin, green = myosin II

Cytokinesis in plants